The Marmorkreb is a variant of the marbled crawfish that is self fertilising -- most craws require a boy and a girl, who fall in love and spend lots of time pinching each other, and then they make little baby crawfish. Marmorkrebs, however, just hide under a rock with a copy of PlayClaw and clone themselves. They're kind of interesting, and fairly well studied.
However, crawfish as a whole can be a major headache when they escape into the wild, which can occur if they're used for bait, if they meander out of your backyard pond, or even if you flush some down the drain (they handle chlorine pretty well). They can devastate an ecosystem, and these guys are bad because it only takes one to start up a breeding population.
There's a lot of talk about restricting the marbled craws (Marmorkrebs) at a federal level, but as of yet, nothing has been done. They are illegal to even posses in Virginia (and, in fact, the purchase, import, sale, or trade of any species of crawfish is illegal in Virginia ... but that's another topic). However, they are perfectly legal to trade, sell, buy, posses, or eat in Maryland. The joy of being in a "tristate" area is that we have to deal with different laws that apply to different people. If someone at a CCA meet were to buy these and then drive home to, say, Chantilly, not that anyone who lives in Chantilly has any, y'know.... they would be committing a rather serious crime, probably without even knowing. However, if the person sitting next to them bought them and took them to Frederick, MD ... they'd be perfectly fine. (As an aside, there are a handful of plants that sometimes show up at CCA auctions that are the same, the most common being Hygro. polysperma. Legal for us Marylanders, not so for Virginians.)
While I personally think the laws in Virginia completely restricting crawfish are a little overbearing, the laws restricting Marms are not -- these buggers have gotten out and caused a lot of problems. As well as outcompeting native craws, destroying aquatic vegetation, eating native fish, outcompeting native fish, etc., they are serious vectors of a couple of diseases that can devastate other aquatic animals.
Oh, and because someone will tell me that you can buy crawfish at any petsmart, so I'm full of it ... this is an issue that has been brought up a NUMBER of times throughout Virginia. Apparently, they are not adequately enforcing the law, though they are gradually stepping this up ... Primarily, they're focused on going after small time traders. There have been stings of people who posted to trading sites (particularly craigslist) where the Va DGAF will arrange a meet to purchase crawfish and then arrest the person who is selling. Ironically, at least one of these made a lot of news because the meeting place arranged was a big chain store that happened to have Electric Blue Lobsters in stock -- which was the species being traded anyhow. Laws ain't fair, who knew?